When database connection fails, display a useful and clean page to user

[mattab]

Currently when the database connection cannot be established, the following message is displayed:

The Piwik configuration file couldn't be found and you are trying to access a Piwik page.
You can install Piwik now
If you installed Piwik before and have some tables in your DB, don't worry, you can reuse the same tables and keep your existing data!

This was a regression introduced in 2.3.0.

The goal of this ticket is to provide all Piwik users a pleasant experience in the tragic time where Piwik instance doesnt' work as expected.

Tasks

• When DB connection fails display a simple error page with function Piwik_ExitWithMessage and showing info such as:
  • Error msg from Mysql eg. "SQLSTATE[2003 Can't connect to MySQL server on 'analytics.db.company.net' (111)"
  • "It may have been a temporary issue, try <a>refresh the page</a>"
  • "If the problem persists please contact your Piwik administrator."
• Add screenshot test for "failed database connection" to check we don't regress

[diosmosis]

In 080829b: Fixes #5400, display useful error message when a database connection cannot be achieved. Deprecating Config.badConfigurationFile event.

[diosmosis]

In 412dc71: Refs #5400, fixing UI tests and regression in Installation process.